A wireless digitizer tablet is basically a circuit board composed of equidistance-spaced antenna loops aligned in a lattice array based on X-Y coordinates. The digitizer tablet is designed to receive electromagnetic waves emitted from a wireless stylus and calculate the absolute coordinates of the wireless stylus. As illustrated in a schematic distribution chart of the antenna loop in a conventional digitizer tablet (in X-axis only) shown in FIG. 1, one end of every antenna loop 10 is connected with a switch 11 while the other end is grounded, wherein the ON/OFF operation of the switch 11 is controlled for detection of a signal induced in each antenna loop 10.
As it is well known however that the magnetic intensity is in inverse proportion to square of distance, hence, the farther the wireless stylus leaves from the antenna loop, the weaker is the signal to be received by the latter and vice versa. Basing on this relationship between the magnetic intensity and the distance, the coordinates of the wireless stylus may be calculated to thereby obtain the exact location of the stylus by analyzing the signal received by each antenna loop after a sequential scanning of all the antenna loops is made by a microprocessor in the digitizer tablet.
In today's high-speed data transmission environments, electromagnetic interference (EMI) has become extraordinarily conspicuous and more serious. For example, under a large scale digitizer tablet, a good number of antenna loops is buried, and for reducing the quantity of the antenna switch, the -shaped antenna loops in relatively longer length are adopted which are more liable to be interfered by EMI, particularly by the EM waves radiated from some computer peripherals, especially the monitor.
Therefore, it is until now a very important issue: “How can people do better to eliminate more efficiently the external EMI of a digitizer tablet?”